The present invention relates to a process for producing a graft copolymer having an elastic trunk polymer and having excellent powder characteristics and low fine powder content.
Graft copolymers each having an elastic trunk polymer are normally produced by emulsion polymerization because of their properties. They are normally obtained in powder form by coagulating graft copolymer particles in latex form with acids or salts and dehydrating and drying the coagulated graft copolymers.
Graft copolymer powders generally have indefinite forms and contain fine particles. Therefore, these powders have poor fluidity and tend to cause handling problems such as (a) blocking of powder particles during storage and (b) plugging of transfer lines due to insufficient fluidity. To keep pace with recent efficiency improvement programs such as automated powder metering and larger scale transportation methods, improvement of powder characteristics of graft copolymers particularly in blocking and fluidity is strongly desired.
Various methods for improving powder characteristics of graft copolymers have been proposed. They include (a) a method in which a copolymer latex is directly spray-dried to convert the powder into particles of spherical form, (b) a method in which the conditions for coagulating or salting-out a copolymer latex are controlled and (c) a method in which a copolymer latex is dispersed in a particular solvent to convert the copolymer into spherical particles, which are then coagulated. However, these methods have not been fully satisfactory in that powder characteristics are not improved sufficiently, or improvements are seen but some of the polymer's inherent properties are lost, or the copolymer manufacturing cost (mainly utility cost) increases. Therefore, no satisfactory improvement method has yet been found. For example, in the spray-drying method, thermal stability of the resulting polymer becomes poor and operational cost such as utility cost increases. In a method in which a latex is dispersed in a particular solvent to convert copolymer molecules into spherical particles (Japanese Patent Laid-open Pub. No. 68285/1977), a large quantity of the solvent must be handled, and, because spherical latex particles are coagulated from the outside, the coagulation becomes non-uniform, and fish eyes may be caused during processing of the product.